At the present time, the practice of deep-hole drilling with the use of twist drills is characterized by low cutting speeds and fast wear of the cutting tool. This is concerned with the fact that with greater drilling depths, difficulties are encountered in supplying coolant to the cutting lips of the twist drill.
Widely known in the present state of the art is a device for the supply of a lubricating-and-cooling agent (hereinafter referred to as "coolant" to the comprising a bath filled with coolant, wherein a workpiece to be machined is accommodated and secured so that the level of coolant is higher than the hole to be drilled (cf., a textbook "Unit-built Machine Tools" by G. I. Melamed et al., 1964, "Mashgiz" Publishers, Moscow, (in Russian).
With low drilling depths, coolant readily flows to the hole being machined, whereby the drill cutting lip are lubricated and cooled down. In deep-hole drilling, the drill operates virtually dry because a steam "cushion" produced at the drill point prevents coolant from getting to the cutting lips, and the twist drill, which operates as a "screw pump", tends to pump coolant out of the hole. The joint action of these factors results in the need to withdraw, at regular intervals, the drill from the hole being machined, thus reducing the machining efficiency.
Known presently is another device for the supply of coolant under pressure to the cutting tool in the machining zone, comprising a housing hermetically sealed by a cover having an opening provided with a seal and adapted to receive an arbor carrying the cutting tool (cf. "Proceedings of Higher Educational Institutes", "Machinostroyeniye" Publishers, Issue No. 12, 1969, Moscow, "Cutting Processes Under High Static Pressures of Coolant" by V. M. Yaroslavtsev, V. V. Sabel'nikov, pp. 172-176 (in Russian). The workpiece to be machined is arranged inside the housing. Coolant under the required pressure is fed by a pump into the housing. High pressure developed inside the housing enables the coolant to penetrate directly to the tool cutting lips, which considerably increases its endurance.
However, the arrangement of the workpiece to be machined inside the pressurized housing involves substantial time wasted on depressurization, removal and positioning of the workpiece to be machined, filling the housing with coolant and its pressurization, which significantly reduces the process efficiency, while the provision of a high-lift pump renders the construction more expensive and sophisticated.
Still another device for supplying a lubricating-and-cooling agent to the cutting tool is known to comprise a housing with an opening adapted to receive an arbor with the cutting tool, a seal mounted on the housing in the zone of interaction between the cutting tool and the workpiece being machined, pipes for the coolant to be fed to and discharged from the housing, and an opening for chip disposal communicating with the housing space (cf. German Patent No. 747,139). The housing is made in the form of a bell resting with its lower portion on the surface of the workpiece being machined.
Coolant is fed into the housing through a coolant supply pipe union turned into the housing. The chip disposal opening and the coolant drain-out opening are arranged above the supply pipe union. The position of the chip disposal opening determines the height of the coolant column. Static pressure produced by the coolant column causes the coolant to penetrate into the holes being machined, thus increasing the cutting tool endurance.
However, the latter device described hereinbefore fails to produce a sufficiently high coolant pressure due to the fact that the housing is not pressurized, and the amount of pressure is only determined by the height of the coolant column which, in turn, is restricted by the housing dimensions. This results in low drilling efficiency. Besides, the device mentioned above is only practicable for vertical drilling.